Various studies have indicated that the susceptibility of plants to certain diseases is not synonymous with the absence of the genetic potential for resistance mechanisms to those diseases. In fact, it is known that resistance can be induced in apparently susceptible plants by inoculation with avirulent forms of plant-pathogens, hypovirulent plant-pathogens or by restricted inoculation with plant-pathogens. The resulting induced resistance is persistent and generally non-specific for a pathogen.
This defense system is a complex interaction of early pathogenic recognition events generating signals which are transduced from the site of inoculation intra- and intercellularly throughout the entire plant. These signals trigger a series of inducible defense reactions with the aim of blocking or even killing the invading pathogen. Many defense reactions are controlled at the gene transcription level.
Pathogen detection takes place as close as possible to the plant surface. Cell-wall degradation products of the attacking pathogen (glucan elicitors) as well as fragments of the plant cell under attack (oligogalacturonide elicitors) are amongst the best described alarm signals. From the site of attack, secondary signals are spread all over the plant. The most documented compound within this signal chain is salicylic acid but electrical signals have also been described as defense inducing signals.
The defense reactions which are activated by incoming alarm signals cover a broad spectrum of chemical, biochemical, and mechanical defense. In monocotyledonous plants reinforcement of cell-walls by callose-deposits opposite to the point where a pathogen tries to penetrate is often observed. Induction of hydrolytic enzymes (e.g. chitinases with lysozyme activity, .beta.-1,3-glucanases, proteases) is observed in di- and monocotyledonous plants. Plants can also react by synthesizing toxic secondary metabolites, so-called phytoalexins, in locally highly elevated concentration, which can kill invading microorganism. One of the earliest reactions of an attacked plant cell is the generation of active oxygen radicals, often a start of the complete sacrifice of a limited number of cells surrounding an infection site.